The invention relates to a semiconductor acceleration sensor and a method of fabrication thereof.
Silicon etching technology instead of the conventional cutting processing has been employed to a fabrication of the semiconductor acceleration sensor. A typical semiconductor acceleration sensor in which air is employed as a damping material will hereinafter be described with reference to the accompanying drawings. Referring to FIG. 1, the semiconductor acceleration sensor comprises a upper stopper 51, a sensor body 52 and a lower stopper 53. The acceleration sensor has a sandwich construction of three layers. The sensor body 52 for sensing an acceleration comprises a middle layer in which a weight 54 made of a silicon and a beam 55 made of a silicon is formed as a monolithic construction. The upper and lower stoppers 51 and 53 are provided on the opposite sides of the middle layer of the sensor body 52 to protect the sensor body 52 in the middle layer and to provide a limitation to a vibration of the weight 54 in the sensor body 52. A depression 56 is also provided in the lower stopper 53 to allow a vibration of the weight. In this case, air is employed as a damping material, because in general air has a suitable temperature characteristic for a damping. Since air is a gas, a viscosity of air is likely to be independent of a temperature. Also, a viscosity coefficient of air is very small. It is therefore necessary to provide a very small clearance, for example, on the order of several microns, between the weight 54 and the lower stopper 53 so as to provide an effective damping to a vibration of the weight 54. In order to suppress a strain caused by a thermal expansion of the stoppers 51 and 53, silicon is preferably selected for the stoppers 51 and 53 as well as the weight 54. Also, a Pyrex glass, which has a coefficient of a thermal expansion near to that of silicon, is available.
Adhesion of the silicon sensor body 52 and the stoppers 51 and 53 of the silicon may be carried out by use of pressure welding of gold. Adjustment of the gold film thickness serving as an adhesive allows control of the clearance so as to realize a preferable damping to the vibration of the weight 54 of the sensor body 52. Adhesion of the sensor body 54 of the silicon and the Pyrex glass stoppers 51 and 53 may be carried out by use of electrostatic bonding method. The lower stopper 53 is provided with a depression by means of etching so that the preferable clearance between the weight 54 and the lower stopper 53 may be available.
An alternate conventional semiconductor acceleration sensor in which silicon oil is employed as a damping material will be described with reference to the accompanying drawings. Referring to FIGS. 2 and 3, since the silicon oil has a very high viscosity as compared with air, satisfactory damping may be provided to the vibration of the weight without providing a small clearance between the weight and the stopper. Since the silicon oil is essentially a liquid, a container for the silicon oil is required, for example, a metal case 62. A sensor body 61 is sealed with the metal case 62 before the silicon oil is fed into the metal case 62 through a silicon oil inlet 63 and a cap for the silicon inlet 63 is put thereon. When the silicon oil is fed into the metal case 62, a release of air from the metal case is required. A blow hole 64 for releasing the air within the metal case 62 is provided on the upper surface of the metal case 62. Further, at a bottom surface of the case, an electrode 65 is provided to receive and deliver an electric signal associated with acceleration.
However, the set forth conventional semiconductor acceleration sensor employs air as the damping material in order to provide damping to the vibration of the weight. In this case, although a temperature characteristic of air as the damping material is superior, the precise setting of a very small clearance between the weight and the stopper is required. It is difficult to fabricate such a semiconductor acceleration sensor.
Further, there is a disadvantage that, when an adhesion operation is carried out to cover the sensor body with the upper and lower stoppers, an needless stress or strain is generated in the sensor body. A height of a rim portion of the sensor body is relied on for providing a clearance between the weight and the stopper. In general, the sensor body being subjected to damping has a different height from that of the rim portion, because the sensor body is subjected to some stress or strain generated on the fabrication of the sensor body. It is difficult to fabricate the acceleration sensors having a predetermined damping characteristic.
Further, in the semiconductor industry, a fabrication process of feeding a silicon oil into a semiconductor device and sealing thereof are not common. In the above sensor using silicon oil, further requirements for the semiconductor acceleration sensor are the metal case 62 and the silicon oil feeding means, for example, the silicon oil inlet 63 and the blow hole 64 for releasing the air within the metal case 62.